Are there any studies that show whether mobile ads convert as well as desktop ads?

I have a lot of mobile traffic, but from my own experience, I never buy anything from mobile. I want to add a -90% negative bid modifier for mobile. Is there any evidence that mobile ads convert better or worse than desktop ads?

It really varies from one niche to the next. I have some niches that get 95% of conversions from mobile traffic, and some that get less than 20%.

What you should do is wait until you have enough data to know what the value per click, or cost per result, is before making any adjustments. Beware, if you are looking at only a day or two of data is typically way too little to make a valid judgement of conversions.

Once you have enough data to make an accurate adjustment you can then do a bid adjustment to levels out the CPA by device.